The heat - and dry - is on

Australia's notoriously variable climate is on full display, with parts of the nation about to experience one of the largest heatwaves in territorial extent in decades after coming off a sharp shift in 2012 from wetter to drier-than-average conditions.

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''We have a major heat event under way,'' Karl Braganza, manager of climate monitoring at the Bureau of Meteorology, said. ''There are not many instances in the historical record where you get a heat event covering such a large area of the continent.''

Brett Dutschke, senior meteorologist at Weatherzone, said it was unusual to have so prolonged a hot spell. ''It's a once-in-20 or 30-year heatwave event in desert areas,'' he said. ''More populated areas further south … are going to experience some of this as well.''

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The mercury is forecast to hit 36 degrees in Melbourne on Thursday and 41 degrees of Friday, with temperatures also soaring in Canberra although Sydney will largely be spared. Adelaide will swelter in 39-degree heat today and 42 tomorrow, the bureau predicts, while even Hobart will experience 32-degree and 38-degree maximums over the two days.

Much of south-eastern Australia has weathered warm temperatures for months. ''We effectively had midsummer heat arriving at the end of spring,'' Dr Dutschke said.

Nationwide, though, 2012 was a year of near-average rain and above-average temperatures, the Bureau of Meteorology said in its annual climate statement released this morning.

Day-time maximum temperatures in the August-December period were the second-highest on record, at 1.58 degrees above average and just 0.16 degrees below the all-time high for the period reached in 2006.

Temperatures for the year as a whole were 0.11 degrees above the 1961-90 average of 21.81 degrees, while the 2003-2012 decade was the fifth-warmest 10-year period on record. Rainfall averaged 476 millimetres, compared with a 465mm average over the 1961-1990 benchmark period.

The rainfall shift during the year, though, was stark. Much of the continent went from cool and wet in the March quarter to drier-than-average conditions within a short spate of time.

"The stunning reversal in rainfall from wet to dry across large parts of the continent, particularly the south, is a result of a switch from a La Nina [weather pattern] early in the year to El Nino-like conditions through winter and spring,” Dr Braganza said.

"Large swings in rainfall, from wet to dry or dry to wet, are not uncommon in Australia, in association with the El Nino - Southern Oscillation," he said. "However it was certainly a talking point in spring 2012 that many locations across the south of the continent returned to the type of dry conditions that typified the Big Dry prior to 2010."

While much of south-eastern Australia enters its high fire danger period, there may be some modest relief on the way. Dr Dutschke says models indicate temperatures and rainfall will start to trend back to more normal summer patterns from about the middle of January.

2012's average rainfall was sharply lower than the 699mm mean recorded during 2011. That year of high rain also meant lower temperatures, with 2011 the only year in the past decade of below-average maximums.

In 2012, maximum temperatures were 0.51 degrees above average while minimums were 0.28 degrees below the norm over the 1961-90 period, the bureau said. In fact, winter last year was the third-coolest on record, with minimum temperatures 0.91 degrees below average.

The gap between maximum and minimum temperatures was the third-largest on record. "The more extreme years of 1964 and 2002 also saw severe drought over most of Australia," the bureau said.